$ oerr ora 12520
12520, 00000, "TNS:listener could not find available handler for requested type of server"
// *Cause: None of the known and available service handlers for requested type
// of server (dedicated or shared) are appropriate for the client connection.
// *Action: Run "lsnrctl services" to ensure that the instance(s) have
// registered with the listener and that the appropriate handlers are
// accepting connections.